1. Field of the Invention
The present invention relates to nickel-hydrogen batteries and cells and more particularly to a method and apparatus for storing such batteries and cells in a way that will minimize internal battery cell degradation.
The method applies equally to both cells with hydrogen stored in pressure vessels and those using metal hydride storage or electrodes.
2. Description of the Prior Art
Experience with nickel-hydrogen batteries and battery cells has indicated that under some conditions of storage, the capacity of the cells can degrade as a result of undesirable chemical reactions. Investigation has revealed that storage at high voltages is also a concern because oxidation of active material oxides and the generation of oxygen causes the nickel-hydrogen batteries or cells to degrade.
However, the condition of primary concern is storage at low voltage. If excess hydrogen is present in the cell it can result in chemical reduction of the active material oxides in the nickel electrode.
To alleviate this condition, one alternative approach provides for storage at low voltages with excess nickel electrode capacity. This approach has several disadvantages. It reduces the cell energy density and allows oxidation of the negative electrode catalysts during storage if the negative electrode potential rises to that of the nickel electrode.
Conventional methods for storing nickel-hydrogen batteries or cells have been drawn directly from experience with nickel-cadmium batteries. These storage methods are either open circuited and discharged or short circuited and discharged. In the open circuited case, the cell will gradually drop to low voltages as a result of catalytic self-discharge processes at the nickel electrode. In the short circuited case, the cell will drop to a low voltage because of the short circuit connection between the cell terminals. In contrast to nickel-cadmium cells, the negative mass in a nickel-hydrogen cell (hydrogen) is in direct contact with the positive mass and slowly reacts to effect complete discharge similar to an external short circuit.
It is therefore an object of this invention to provide a method for storing nickel-hydrogen batteries and cells that precludes all of the degradation modes outlined above, as well as all other degradation modes known to result from storage.
It is also an object of this invention to provide an apparatus for applying this storage method to both nickel-hydrogen batteries and battery cells.